material that does not let any liquid pass through it (11)

water sources that are protected from contamination (1)

an emergency situation such as a burst pipe (8)

biological species that gives information about the environment (2)

micro-organisms and viruses that can invade the human body and cause disease (2)

material that does not originate from living organisms (4)

people who have been forced to flee to another part of their own country due to factors beyond their control (14)